The present disclosure relates generally to an exposure system and method, and particularly to an exposure system and method that compensates fine-tuning values of overlay correction parameters for an adjusted exposure tool in semiconductor manufacturing.
Photolithography is an important technology in semiconductor manufacturing. The number of masks used in photolithography corresponds to the complexity of a manufacturing process. Photolithography affects structures of semiconductor devices, such as patterns of layers and doped regions, and determines the functional effectiveness thereof. Since photolithography is complicated, the exposure tool performing the photolithography can become a bottleneck in the manufacturing process.
Exposure of wafers is implemented in a “Step and Repeat” fashion to transfer high resolution patterns to the wafers. A pattern on the mask is projected and sized to one portion or block of the wafer. This is repeatedly implemented for all blocks on the wafer individually until the entire wafer is exposed. Since only the pattern of one layer is transferred to the wafer after each block is exposed, and there are many patterns of layers and corresponding masks involved in one manufacturing process, piece alignment between the blocks of the wafer, and overlay alignment between the patterns of the layers, are essential to processing. Additionally, performance and baseline of an exposure tool varies slightly with time. For a precise and accurate exposure, the processed wafers are measured to compensate the overlay correction parameters used by the exposure tool. The parameters, once compensated, are used by the exposure tool to process subsequent wafers.
Conventionally, a run to run system calculates fine-tuning values according to the last run of wafers, and compensates the parameters using the fine-tuning values. However, the system is designed for parameter adjustment between lot wafers processed by the exposure tool with time. If the exposure tool encounters malfunction or failure, or undergoes routine maintenance adjustment, the original fine-tuning value for overlay correction is not usable since the equipment baseline is changed by the adjustment. Since there is no effective mechanism of parameter compensation for the adjusted exposure tool, the tuning operation is implemented manually.
FIG. 1 shows an example of conventional fine-tuning value adjustment. Equipment baseline B is gradually shifted with time. Prior to adjustment at time TPM, four products are processed by the equipment tool, and corresponding fine-tuning values are F1, F2, F3 and F4. In this case, since the equipment baseline B is shifted by offset d at time TPM, each fine-tuning value F1, F2, F3 and F4 is manually adjusted with the offset d as fine-tuning values AF1, AF2, AF3 and AF4. However, since each fine-tuning value is generated based on a different equipment baseline, much of the adjusted fine-tuning value is far from real process conditions or real equipment conditions. Sometimes, pilot wafers are processed by the exposure tool and then measured to obtain the compensation with some approximate calculations.
Since there may be a large number of product types in an IC foundry, conventional parameter compensation can be time-consuming and increase rework rate, thereby resulting in mistakes, and decreasing equipment availability and throughput of the manufacturing process.